ISLAMABAD (AP) -- A cleric who has fired up Pakistanis angry at perceived government corruption and indifference demanded the country's political leaders resign in a speech to thousands of his supporters who amassed in the capital early today and clashed briefly with security forces.

The dramatic entry into Pakistani politics of Tahir-ul-Qadri, a preacher who until recently lived in Canada, has sparked concern from some that he is seeking to derail elections at the behest of the powerful army. Polls are expected this spring.

Qadri has denied that and insisted his vague demands for election reform are simply meant to root out corruption in the political system. He pledged several weeks ago to lead a "million-man march" on Islamabad to press his demands.

During a 40-minute speech delivered behind bullet-proof glass in the early morning, Qadri told his supporters that the government's mandate was finished.

"I give you time until tomorrow to dissolve national and all four provincial assemblies otherwise the nation will dissolve them on their own," he said. He vowed to address his followers later in the morning in front of the parliament building.

Qadri called on the demonstrators to break through the containers blocking them from the government offices and peacefully march toward the protected enclave that is often called the "red zone" in Islamabad.

Following his cry, some of the marchers pushed aside the shipping containers that had been placed on the street to block them and walked toward the enclave. There another row of shipping containers and a heavy police presence blocked them from going any further and the protesters appeared to stop.

The rally was widely peaceful until police and protesters clashed briefly in the early morning.

Television footage showed police shooting into the air to push back protesters and a man on the ground being beaten by what appeared to be protesters. Demonstrators threw rocks at a vehicle, while others held up shells and an empty tear gas canister.

Helicopters could be heard circling overhead.

Minister of Interior Rehman Malik, speaking on Pakistani television, said some of the demonstrators had weapons and attacked police with stones.

"Tahir-ul-Qadri has said that the marchers would remain peaceful but police have been stoned and shots were also fired," he said.

In an email to The Associated Press, a spokesman for Qadri, Shahid Mursaleen, blamed the entire incident on the security officials and said police had opened fire unprovoked.

Thousands of people remained on the streets after the clashes stopped. Qadri put the crowd assembled on the main avenue leading to the government center at 4 million but far fewer were actually in attendance. One city official put the number of protesters at roughly 30,000. He did not want to be identified because he was not authorized to speak to the media.

Many in the crowd waved green and white Pakistani flags and wore buttons emblazoned with the cleric's picture.

Qadri has called for vaguely-worded reforms to the electoral system such as making sure candidates for office are free of corruption. His words have inspired many Pakistanis who are frustrated with a government that they say has given them nothing but unemployment, electricity blackouts, and terror attacks as its five-year term comes to an end.

"There is no electricity and no gas, and the government has done nothing," said Faizan Baig, a 23-year-old pharmaceutical company worker who traveled to Islamabad from the northwest town of Abbottabad. "Qadri feels pain for the people, while the government feels no pain for the people."

Security was heavy throughout the city, although the rally appeared to be largely peaceful. Thousands of police in riot gear protected the streets, and cell phones were jammed after the government warned that militants were planning to attack the protesters.

Qadri returned to Pakistan in December after years in Canada, where he's also a citizen. He heads a religious network in Lahore and gained some international prominence by writing a 2010 fatwa, or religious opinion, condemning terrorism.

But he was never a national political figure until this winter, when his calls for reforms ahead of elections galvanized many Pakistanis disenchanted by the existing parties.

His arrival in Islamabad was met with raucous cheers, and supporters showered his black SUV with rose petals.

Qadri also asked his supporters to take the security of the capital in their hands and guard and protect each of the buildings of Islamabad. The cleric took an oath in front of the crowd that they all will remain peaceful but stay in Islamabad until the revolution is completed.

"They are no more rulers but former rulers. Don't follow their orders! I have come here to get you out of their slavery," he said.

Many of the protesters had blankets and appeared ready to camp.

Some of Qadri's comments have sparked concern that the cleric is being uses as a front for the Pakistani military to disrupt the democratic process just as the country prepares for a historic transfer of power from one civilian government to another.

He has called for a military role in picking the caretaker government that will take over temporarily ahead of elections and has said it could stay in place longer than normal to enact necessary reforms.

Those comments, as well as questions about the origins of his funding, have sparked fears Qadri is really trying to derail the upcoming vote on behalf of the military, which is believed to dislike both the main political parties vying for power, and pave the way for a military-backed caretaker to hold power indefinitely. Qadri has denied any such involvement.